Quadrature amplitude modulation is a modulation scheme utilizing two carrier waves, usually sinusoids, which are out of phase with each other by 90°. The amplitude of the two waves, 90° out-of-phase with each other (e.g., in quadrature) can be changed (e.g., modulated or keyed) to represent a given data signal. Amplitude modulating two carriers in quadrature can be equivalently viewed as both amplitude modulating and phase modulating a single carrier.
Quadrature amplitude modulation can be performed by splitting a flow of bits to be transmitted into two equal parts to generate two independent signals to be transmitted. These signals are encoded separately and in one signal (e.g., in an “in phase” channel) is multiplied by a cosine, while the other signal (e.g., in a “quadrature” channel) is multiplied by a sine, thus producing two signals with a phase difference of 90°. These in phase and quadrature signals can be added together, amplified, and transmitted through the real channel.
Pre-distortion is a technique used to improve the linearity amplifiers, such as radio transmitter amplifiers. Pre-distortion inversely models the amplifier's gain and phase characteristics and, when combined with the amplifier, produces an overall system that is more linear and reduces the amplifier's distortion. In essence, inverse distortion is introduced into the input of the amplifier, thereby cancelling any non-linearity the amplifier might have. Calculation of the pre-distortion can rely on matching a copy of the (clean) input signal with the (distorted) amplified signal and then determining how best to pre-distort. To make the signal comparison meaningful, the input and the amplified signals can be time-aligned—so that the differences in the two signals is due to distortion and not simply due to sampling the signal at a different point in time.